The present invention refers to a non impact serial-parallel or parallel dot-matrix printer of the type in which a single row of regularly spaced dot-size printing elements is positioned parallel to the printing line of the recording medium.
In the serial parallel printer the row is reciprocated in the printing line direction, the recording medium is moved incrementally along a direction perpendicular to the printing line in synchronism with the movement of the row and control means are provided for selectively energizing the printing elements at a succession of positions of the row in each one of a succession of strokes thereof, such that each printing element prints all the dots of at least one character of the line during the succession of strokes.
In the parallel printer the row of printing elements is held stationary and parallel to the printing line of the recording medium while only the reconding medium is incrementally advanced perpendicularly to the printing line direction, and each printing element is selectively energized for printing all the points of a matrix column of a printable character of the line.
U.S. Pat. No. 3,951,247 discloses a serial-parallel thermal dot-matrix printer of the above mentioned type in which the row of printing elements is supported in a single thermal head including a ceramic insulating planar substrate having coated thereon a row of resitive printing elements and a pattern of conductors for selectively passing heating current through each one of the resistive elements.
The use of a single head for supporting the row of printing elements does not cause any drawback when the length of the printing line does not exceed a certain number of characters, typically 16 characters.
But when the printing line is very long, for instance 80 characters, the construction of a single planar ceramic substrate extending along almost all the length of the printing line, for supporting a row of a 80 printing elements, in the case of serial-parallel printer, or 80 .times. N printing elements in the case of a parallel printer (where N is the number of columns of the matrix of dots according to which the characters are printed), is very critical, since it is difficult to obtain the ceramic substrate perfectly planar and little inperfections in the planarity of the substrate or in the linearity or cylindricity of the platen will cause a non uniform contact of all the printing elements against the recording medium and consequently characters printed on the same line have a non uniform darkness.
This type of drawback is not limited to thermal printers but in general to all the non impact printers using a planar head carrying thereon dot-size printing elements which should contact uniformly the recording medium for a correct printing operation. Electrosensitive or electrostatic planar heads have the same drawbacks when the row of printing elements become too long.